Abstract [en]

Recent studies in humans have suggested sex differences in venouscompliance of the lower limb, with lower compliance in women.Capillary fluid filtration could, however, be a confounder inthe evaluation of venous compliance. The venous capacitanceand capillary filtration response in the calves of 12 women(23.2 ± 0.5 years) and 16 men (22.9 ± 0.5 years)were studied during 8 min lower body negative pressure (LBNP)of 11, 22, and 44 mmHg. Calf venous compliance is dependenton pressure and was determined using the first derivative ofa quadratic regression equation that described the capacitance-pressurerelationship [compliance = 1 + (2·2· transmuralpressure)]. We found a lower venous compliance in women at lowtransmural pressures, and the venous capacitance in men wasincreased (P < 0.05). However, the difference in compliancebetween sexes was reduced and not seen at higher transmuralpressures. Net capillary fluid filtration and capillary filtrationcoefficient (CFC) were greater in women than in men during LBNP(P < 0.05). Furthermore, calf volume increase (capacitanceresponse + total capillary filtration) during LBNP was equivalentin both sexes. When total capillary filtration was not subtractedfrom the calf capacitance response in the calculation of venouscompliance, the sex differences disappeared, emphasizing thatvenous compliance measurement should be corrected for the contributionof CFC.

Lindenberger, Marcus

Abstract [en]

Acute haemorrhage is a leading cause of death in trauma. Young women (YW) seem more susceptible to hypovolemic circulatory stress than young men (YM), but the underlying mechanisms are not clear. Elderly subjects are more vulnerable to haemorrhage, with a decreased defence of central blood volume in elderly men, but the defence has not been evaluated in elderly women (EW). The aims were to assess differences in cardiovascular responses to hypovolemic circulatory stress, emphasizing compensatory mechanisms to maintain central blood volume in YW, EW and in women prone to vaso‐vagal reaction (VW).

Lower body negative pressure (LBNP) was used as a model for haemorrhage and to create acute hypovolemic stress. Volumetric techniques were used to assess venous compliance, capacitance and capillary fluid exchange both caused by LBNP in the calf and the response to maintain central blood volume.

LBNP induced a comparable hypovolemic stimulus in YW and YM, with lower calf venous compliance and capacitance but higher net capillary fluid filtration in YW. YW responded with smaller vasoconstriction without association between P‐NE and peripheral vascular resistance in contrast to YM. Venous capacitance response was decreased with time in YW. Further, net capillary fluid absorption from peripheral tissues to central circulation was decreased in YW in response to hypovolemic stress. All in all, this indicates less efficiency to defend central blood volume in young women.

LBNP induced a slower hypovolemic stimulus in VW compared with nonvagal women. Further, the cardiopulmonary baroreflex was less efficient, and the venous capacitance response from peripheral tissues to central circulation was decreased, which may explain their susceptibility to orthostatic challenge.